Melt Inclusions in Basaltic and Related Volcanic Rocks

Abstract

Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems (Sorby 1858; Roedder 1979, 1984). After trapping, melt inclusions are potentially isolated from external melt and thus provide a way to investigate melts trapped during magmatic evolution—driven by processes such as crystal-liquid separation, vapor saturation and degassing, magma mixing and assimilation—which can dramatically alter the compositions of the eventual erupted (or intruded) magmatic end products. Melt inclusions are a powerful tool for the study of basaltic magma systems and their mantle source regions, and are widely used to study the origin and evolution of mantle-derived magmas. Melt inclusions have specific uses in the study of volatile elements (see chapters by Metrich and Wallace 2008, Moore 2008, and Blundy and Cashman 2008), but also provide unique information about the range of melt compositions present within basaltic magmatic systems, and how these reflect mantle sources and the processes that occur during melt generation, evolution, transport and eruption. This review outlines techniques used to obtain chemical and other information from melt inclusions, discusses the processes which lead to melt inclusion trapping in phenocryst minerals, examines the possible means by which melt inclusion compositions might be fractionated during trapping or during subsequent re-equilibration with the host mineral or external melt, and discusses some implications of melt inclusion compositions for the nature of basaltic melt generation and transport systems. This review is largely restricted in scope to studies of volcanic rocks of basaltic and related composition. This refers to rocks erupted as lavas or tephra with broadly basaltic compositions: SiO2 ~45–52 wt%, relatively high MgO and FeO, and typically containing one or more of the following minerals …